How would you translate a range of ages, e.g. 25 to 60? One English-Latin dictinary (Routledge) suggests ordo and series, and I am leaning toward the latter. I am rather unsure as I am just beginning with prose composition.

I don't quite understand, how altrinsecus is properly used. According to the dictionary altrinsecus (the first time I have heard of this word) means "on/from the other side".

In the first example, qui altrinsecus 25 et 60 annos nati sunt, I guess that annorum is understood after 25, isn't it? So it should read completely qui altrinsecus 25 annorum et 60 annos nati sunt. Or am I wrong (altrinsecus seems to be be able to govern both the accusative and genetive)?

And how do we make sure that when using altrinsecus the right "side of which" is understood? In the second example, qui 25 et 60 annorum altrinsecùs sunt, couldn't it just as well mean "those who are younger than 25 and older than 60 years" (on the other side of 25 and 60 years)?

Yes, I am not certain at all. Ainsworth, however, has it "between the one and the other"; others have it also "from both sides" (post classically). It's an adverb so it doesn't take a case and you construe the sentence accordingly: I guess ,"ages from within the sides[/flanks] of twenty five and sixty". Of course, maybe it's the wrong thing to say there. What about "series annorum inter x et x interjacentum"?